1. Field of the Invention
This invention relates to electronic circuits and, more particularly, to low power supply Bandgap Reference (BGR) circuits used to generate reference currents and reference voltages on a semiconductor device with high accuracy using small gate area, low voltage devices in the analog blocks.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Virtually all systems that manipulate analog, digital or mixed signals, such as analog-to-digital and digital-to-analog converters, rely on at least one reference voltage as a starting point for all other operations in the system. Not only must a reference voltage be reproducible every time the circuit is powered up, the reference voltage must remain relatively unchanged with variations in fabrication process, operating temperature and supply voltage.
A Bandgap reference (BGR) circuit is one manner in which a relatively stable reference voltage may be generated. As explained in more detail below, BGR circuits rely on the predictable variation with temperature of the bandgap energy of the underlying semiconductor material. There are generally two types of BGR circuits, referred to herein as “voltage adding” and “current adding” BGR configurations.
FIG. 1 illustrates an exemplary block diagram of a voltage adding Bandgap reference circuit 100. In general, BGR circuit 100 is configured for producing a reference voltage (VREF) as a weighted sum of two voltages: V1, which is proportional to absolute temperature (PTAT), and V2, which is complementary to absolute temperature (CTAT). As shown in FIG. 1, the reference voltage may be expressed as:VREF=α1*V1+α2*V2  (1)where V1 has a positive temperature coefficient (TCPOSV), V2 has a negative temperature coefficient (TCNEGV) and α1, α2 are non-dimensional coefficients chosen to minimize temperature-dependent variations in the reference voltage across a specified range of temperatures.
Voltage adding BGR circuit 100 may be used for generating a reference voltage, which exhibits relatively little variation across a defined range of temperatures, process corners and supply voltages. As shown in FIG. 2, for example, circuit 100 may provide a relatively constant reference voltage (VREF) across a defined range of temperatures (T−x, T+x), if the coefficients α1, α2 are chosen such that there is a temperature, T0, for which:d(VREF)/dT=α1*TCPOSV+α2*TCNEGV=0 at T=T0  (2)where T is the absolute temperature (K) and T−x<T0<T+x. In other words, (T−x, T+x) defines the range of temperatures for which voltage adding BGR circuit 100 is intended to operate.
In some cases, the negative temperature coefficient voltage (V2) may be generated by developing a voltage across a forward-biased P-N junction diode. In other cases, V2 may be generated by diode-connecting a bipolar junction transistor (BJT), such that the base-emitter voltage (VBE) drop is the voltage that exhibits bandgap behavior. As used herein, the term “diode” may refer to any diode-like element (including diodes, BJTs and CMOS transistors operating in the subthreshold region), which exhibits a diode voltage drop.
In some cases, the positive temperature coefficient voltage (V1) may be generated by subtracting the voltages developed across two P-N junction diodes or two bipolar junction transistors (BJTs). For example, the PTAT voltage can be generated as: 1) the difference between the forward voltages of two P-N junction diodes operating at different current densities, or 2) the difference between the base-emitter voltages (VBE) of two bipolar junction transistors (BJTs) biased in normal active mode of operation, with the two respective base-emitter junctions having different current densities.
In one example, the two forward biased P-N junction diodes (or two BJTs) may be configured to operate at different current densities by constructing the diodes, such that a ratio between the areas of the diodes is N. The ratio (N) between the areas of the two diodes (D1, D2) is usually implemented by replicating the first diode (D1) a number of times (N) to generate the second diode (D2) with N times larger area.
Voltage adding BGR circuit 100 represents an effective technique for obtaining a reference voltage of about 1.25 volts given a supply voltage of a few volts (e.g., about 3 to 5 volts). However, the functionality of circuit 100 tends to suffer (and sometimes fail) under low power supply conditions (e.g., power supply voltages of about 1.6 volts and below, depending on technology). In addition, circuit 100 provides only one reference voltage output (around 1.25 volts), and therefore, cannot be used when more than one reference voltage, a different reference voltage, or a reference current is desired.
Therefore, current adding BGR circuits are sometimes used in place of voltage adding BGR circuits to overcome the disadvantages associated therewith. For example, current adding BGR circuits are often preferred over voltage adding BGR circuits for their ability to: (a) operate under low power supply conditions (e.g., 1.6 volts and below), (b) provide multiple reference voltage outputs simultaneously (including those other than 1.25 volts), and (c) generate both reference voltage and reference current outputs at the same time.
FIG. 3 illustrates one manner in which a stable reference voltage (VREF) may be generated by creating a reference current and then passing it through a resistor. For example, current adding BGR circuit 300 may be used to generate a reference current (IOUT) as a weighted sum of two currents: I1, having a positive temperature coefficient (TCPOSI), and I2, having a negative temperature coefficient (TCNEG1). The reference current (IOUT) may be expressed as:IOUT=β1*I1+β2*I2  (3)where I1 is the PTAT current, I2 is the CTAT current, and β1 and β2 are non-dimensional coefficient values chosen to minimize temperature-dependent variations in the reference current across the specified range of temperatures.
As shown in FIG. 3, a reference voltage (VREF) may be generated by passing the reference current (IOUT) generated by circuit 300 through a resistor of value R such that:VREF=R*IOUT  (4)As in the previous circuit, the reference voltage VREF may demonstrate a relatively small variation (i.e., a small ΔVREF, as shown in FIG. 2) over a specified range of temperatures (T−x, T+x), if temperature-dependent variations in IOUT are minimized. For example, the temperature coefficient of resistor R is one factor, which plays an important role in defining the variation of VREF with temperature. Additional factors will be discussed in more detail below. In some cases, a small variation of VREF with temperature may be obtained by selecting appropriate values for the coefficients β1 and β2 in equation (3), so that the derivative of the reference voltage (VREF) will be:d(VREF)/dT=0 at T=T0  (5)where T is the absolute temperature (K) and T−x<T0<T+x. As before, (T−x, T+x) defines the range of temperatures for which current adding BGR circuit 300 is intended to operate.
Unfortunately, current adding BGR circuits are notorious for their sensitivity to process-induced mismatch between circuit elements, which are otherwise intended to be identical (i.e., matched). For example, process-induced mismatch may occur during fabrication of a semiconductor device, causing otherwise identical devices (e.g., two PMOS transistors with identical gate areas, dopant concentrations, etc.) to exhibit substantially different threshold voltages and drain currents. Process-induced mismatch adversely affects Bandgap operation by shifting the reference voltage output and/or the temperature coefficient of VREF.
In order to compensate for process-induced mismatch, some circuit designers have opted to use large, high voltage devices (with thick gate oxides and large gate areas) in the analog blocks of a Bandgap circuit to reduce gate leakage. Although the thick oxide (e.g., tox≈60 Å) of high voltage devices allows for virtually zero gate leakage, the use of high voltage devices produces a relatively large layout area, considerably increases the design effort and severely limits the overdrive of the matched transistors (especially when coupled with power supply specifications of about 2.0 volts and below). The exclusive use of high voltage devices also renders the approach unsuitable for low power supply voltages (e.g., 1.6 volts and below).
In order to meet low power supply specifications, other circuit designers have opted to combine large, low voltage devices with the use of dummy structures to compensate for process-induced mismatch. However, the thin gate oxides (e.g., tox≈16 Å) and large gate areas (e.g., about 100 to 500 μm2) of the low voltage devices tend to significantly increase the gate leakage problem. In some cases, the amount of gate leakage attributable to the low voltage devices is comparable to the drain operating point current—a level which cannot be accurately controlled or compensated using dummy structures. In addition to uncontrollable gate leakage, the use of large, low voltage devices and dummy structures also results in a relatively large layout area.
Therefore, a need remains for a current adding BGR configuration capable of high accuracy, low power operation. In a preferred embodiment, high accuracy and low power specifications could be met by avoiding the use of large gate area devices within the analog blocks of the BGR circuit.